In the following text, the circumferential, axial and radial directions respectively denote a direction tangential to the tread surface of the tire in the direction of rotation of the tire, a direction parallel to the rotation axis of the tire and a direction perpendicular to the rotation axis of the tire. “Radially on the inside or, respectively, radially on the outside” means “closer to or, respectively, further away from the rotation axis of the tire”. “Axially on the inside or, respectively, axially on the outside” means “closer to or, respectively, further away from the equatorial plane of the tire”, the equatorial plane of the tire been the plane passing through the middle of the tread surface of the tire and perpendicular to the rotation axis of the tire.
A tire comprises a tread which is intended to come into contact with the ground by way of the tread surface and is connected by way of two sidewalls to two beads that provide the mechanical connection between the tire and the rim on which it is mounted.
A radial tire also comprises a reinforcement, comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement, radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of the agricultural type generally comprises at least one carcass layer consisting of very often textile reinforcing elements that are coated with a polymeric material of the elastomer type.
A carcass layer is referred to as turned-up when it comprises a main part that connects the two beads together and is wrapped, in each bead, from the inside of the tire to the outside around a circumferential reinforcing element which is very often metallic and coated in at least one generally elastomeric or textile material, so as to form a turn-up having a free end. The assembly formed by the circumferential reinforcing element and its coating material is usually known as a bead wire.
In the case of a turned-up carcass layer, the turn-up, in each bead, allows the turned-up carcass layer to be anchored to the bead wire. The portion of bead wire in contact with the turned-up carcass layer contributes, in particular upon inflation, to reacting tensile forces in the turned-up carcass layer by coupling. This contribution to reacting tensile forces depends on the torsional stiffness of the bead wire and on the geometry of the turn-up. In the usual case of high torsional stiffness of the bead wire, the tensile forces upon inflation are essentially reacted by the bead wire, with a secondary contribution by the turn-up. In the case of lower torsional stiffness of the bead wire, the tensile forces are reacted both by coupling to the bead wire and by shearing between the turn-up and the materials adjacent thereto, this requiring a sufficiently long turn-up that is to say one in which the end is sufficiently far away radially from the radially outermost point of the bead wire.
A carcass layer is referred to as non-turned-up when it consists only of a main part that connects the two beads together without being wrapped around a bead wire.
In the case of a non-turned-up carcass layer, each of the two end portions of said non-turned-up carcass layer may be coupled either to the turn-up of at least one turned-up carcass layer or to the main part of at least one turned-up carcass layer. Coupling means a region of overlap between the non-turned-up carcass layer and a turned-up carcass layer, allowing tensile forces to be reacted by shearing. In the case of a carcass reinforcement comprising two turned-up carcass layers and at least one non-turned-up carcass layer, each end of a non-turned-up carcass layer may be interposed between the respective turn-ups of two turned-up carcass layers, that is to say it has, on each of its axial faces, a region of overlap with a turned-up carcass layer turn-up.
The reinforcing elements of the main part of a turned-up or non-turned-up carcass layer are approximately parallel to one another and form, with the circumferential direction, an angle of between 85° and 95°. The reinforcing elements of a turn-up of a turned-up carcass layer form an angle, with respect to the circumferential direction, of between 75° and 105°.
A tire for a self-propelled agricultural vehicle, being designed for a recommended load, is conventionally inflated to a recommended inflation pressure of between 2.4 and 3.2 bar. The recommended loads and inflation pressures, for a given tire dimension, are defined for example by the European Tire and Rim Organization (ETRTO) standard.
A tire for a self-propelled agricultural vehicle is referred to as having high bending when it has a loading capacity at least equal to 1.2 times the loading capacity recommended for a standard-technology tire of the same size, known as a reference tire, for a pressure equal to the pressure of the reference tire. It allows three types of use: under overload with respect to the standard-technology tire, or at under-pressure with respect to the standard-technology tire, or with any intermediate combination. The advantage of use under overload is that the productivity of harvesting operations with the agricultural vehicle is increased. The advantage of use at under-pressure is that the compaction of the ground and damage to crops as the agricultural vehicle passes is decreased. Under these use conditions with high bending, it has been found that the mechanical endurance of the carcass reinforcement is substantially reduced compared with use under a load equal to the recommended load combined with an inflation pressure equal to the recommended inflation pressure.
The bending of a tire is commonly characterized by a relative deflection, defined as the ratio in the variation of the radial height of the meridian section of the tire to half the difference between the outside diameter of the tire and the maximum diameter of the rim measured on the rim flange when the tire passes from an unladen inflated state to a laden inflated state. The outside diameter of the tire is measured statically in an unladen inflated state.
The document WO2004106089 describes a tire for an agricultural vehicle of the tractor type that operates with high bending, characterized by a high deflection, greater than 28%, resulting from use at a reduced inflation pressure compared with the recommended inflation pressure, in order to reduce the compaction of the ground and damage to the crops as the agricultural machine passes. According to the invention, the tire is such that the aspect ratio H/S of the height H of the tire to the maximum axial width S of the tire is less than 0.75 and the ratio A/B of the tread width A to the deflection height B of the crown is greater than 17. This document shows that an appropriate choice of the geometry of the tire thus allows use with high bending.
However, such a solution is difficult to apply to a tire for a self-propelled agricultural vehicle of which the geometric dimensions cannot necessarily be adapted as recommended in the document WO2004106089.
Moreover, a tire for a self-propelled agricultural vehicle is essentially designed to have a high loading capacity, whereas a tire for a vehicle of the tractor type, as described in the document WO2004106089, is designed to be able to transmit a high tractive torque to the rim. Since the respective uses of these two types of tires are thus different, the design, and in particular the geometric characteristics, are not easily transposable from a tire for a vehicle of the tractor type to a tire for a self-propelled agricultural vehicle.
Consequently, an alternative solution is necessary to solve the problem of using a tire for a self-propelled agricultural vehicle having high bending.